I am receiving the following when sending to a GMail address.

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Error Type: SMTP
Remote server (142.251.2.27) issued an error.
hMailServer sent: .
Remote server replied: 550-5.7.26 This message does not pass authentication checks (SPF and DKIM both
550-5.7.26 do not pass). SPF check for [thesmiffs.me.uk] does not pass with
ip:
550-5.7.26 [162.216.242.35].To best protect our users from spam, the message
550-5.7.26 has been blocked. Please visit
550-5.7.26 https://support.google.com/mail/answer/81126#authentication for more
550 5.7.26 information.
185-20020a250bc2000000b007b896757b12si33862338ybl.576 - gsmtp



Dynu Mail Server
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Can you advise how to resolve this?

Thanks

Chris